Fix JSON format in some controller actions

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There are some controller actions rendering strings instead of JSON. We should do render json: {error: 'any string'} instead of render json: 'any string'`

app/controllers/concerns/uploads_actions.rb:    render json: "Error uploading file", status: :internal_server_error
app/controllers/projects/merge_requests_controller.rb:      render json: '', status: :no_content
app/controllers/admin/users_controller.rb:        format.json { render json: 'There was an error removing the e-mail.', status: :bad_request }
app/controllers/application_controller.rb:      format.js { render json: '', status: :not_found, content_type: 'application/json' }
app/controllers/concerns/uploads_actions.rb:          render json: 'Invalid file.', status: :unprocessable_entity

The following discussion from gitlab-ce!21774 should be addressed:

• @stanhu started a discussion: (+1 comment)

  That's not valid JSON either:

  irb(main):002:0> JSON.parse('Invalid JSON')
  JSON::ParserError: 765: unexpected token at 'Invalid JSON'

  I'd suggest either: { error: 'Invalid UTF-8'} or just {}.

  Can we add a spec please?